1. The Field of the Invention
The present invention pertains to a method and apparatus to determine the presence of a movable vehicle or pig in a pipeline and, in particular, to identify changes in the acoustic signature of a pig.
2. The Prior Art
Devices which are propelled through pipelines by the fluid flow in the line are called "pigs" and are most often used to clean and/or inspect the pipe or to separate different batches of fluid flowing through the pipe. It is often useful to know when a pig has passed a particular point in the pipe and, if possible, the condition of the pig. There are several methods and devices presently used which are capable of determining the passage of a pig. For example, a mechanical device is attached to the pipe with an actuator portion thereof extending into the pipe. The passing pig trips this actuator through physical contact. The device then actuates mechanical indicator, such as a flag, or an electrical indicator, such as a switch, to indicate that the pig has passed that point. Other devices comprise magnetic, acoustic or electromagnetic sources attached to the pig and a separate detector (or receiver) responsive to the source and located inside or outside the pipe. The detector responds to the signal, or magnetic field, from the passing pig and actuates an indicator.
The ideal pig detector would not only determine the passage of the pig, but the condition of the pig. For example, is the pig in need of repair or has the pig scraped up enough debris from inside the pipe that it should be removed and cleaned and/or repaired. This detector should be capable of use in extreme temperatures, pressures and hostile chemical environments while providing accurate results. The construction of this detector would be such that it would be simple and suitable for field applications as well as usage in the laboratory.
An additional example of the prior art involves the use of ultrasonic techniques for fluid characterization applications. These types of systems can be intrusive or non-intrusive, depending on the application. For example, composition of two-phase fluids can be investigated using an intrusive transit time method, as described in U.S. Pat. No. 5,115,670. This method contemplates the measurement of the transit time of a sound wave between an ultrasonic source and a detector located diagonally across a pipe. In principle this transit time can be used to calculate the speed of sound in two-phase flow. This allows the calculation of the mixture's linear velocity and composition. These quantities allow the calculation of mass flow rates or the energy flow rates. The calculated results and their accuracy, for example steam quality, may depend on separate fluid property correlations. The fluid's chemical composition may effect the sensor's longevity.
Fluid velocities can be obtained using Doppler flow meters (see U.S. Pat. No. 5,115,670). These ultrasonic devices can be non-intrusive (externally mounted in the pipe) and protected from the environment. The idea behind these devices is that an ultrasonic signal is continuously transmitted into a pipe containing fluids where scattering occurs from suspended solids, air bubbles, discontinuities or disturbances in the flowing stream. The scattered signal is detected and its frequency is compared to the transmitted frequency. The difference in these frequencies is proportional to the fluid's velocity. These measurements are considered most accurate when evaluating fluids with Newtonian flow profiles and containing suspended particles or air bubbles.
Generally, the designs of existing flow measurement systems using nuclear, acoustic or electromagnetic methods only address a few of the idealized capabilities and concentrate on measuring a restricted set of parameters while actively probing the medium of interest. These measurement systems can be intrusive or non-intrusive and some may require a side stream sample to obtain the required data. Examples of some of the active acoustic flow measurement systems can be found in U.S. Pat. Nos. 4,080,837; 4,236,406; and 4,391,149.
Passive types of measurement techniques in pipes, specifically simple detection of acoustic emissions or "listening," are available, but are limited in scope and applications. For instance, acoustic emissions can be used to detect: slug flow and the presence of sand in multi-phase pipelines (see U.S. Pat. No. 5,148,405); leaks in natural gas pipelines (see U.S. Pat. No. 5,117,676); and steam quality when the acoustic emissions are obtained from a calibrated steam jet produced by an orifice (see U.S. Pat. No. 4,193,290). The use of acoustic emissions as a passive and non-intrusive method in quantitative characterization of multi-phase flow in pipes appears to be novel.